x13 - thirteen separate trades have been rolled up into this line. when trades occur in quick succession, they are aggregated due to IRC message rate limits. when a quote contains this marker, the volume (see next item) is cumulative for all aggregated trades, and the price (see item after that), is a weighted average per-coin price for this group of trades.

49.9678 - volume of the trade in BTC. 49.9678 total bitcoins have been traded in this group of 13 trades.

6.3103886 - price per bitcoin, in units of whatever currency the trade is in (see next item). for aggregated lines, which this is an example of, the price is a weighted average price per coin (i.e., total number of USD, divided by total number of coins)

USD - currency against which the trade was conducted. see Currency abbreviations section for detail. In this case, trade is for US dollars.